Transmissions are used in motorized vehicles to transmit the engine power to the propelling system of the vehicles (i.e., wheels, propellers, etc.). Various types of transmissions adapt to the different engines and motors in order to propel the vehicle. An internal combustion engine, for instance, does not behave like an electric motor. An electric motor evolves between a full stop to high running speeds at high efficiencies. Therefore, a transmission may couple the electric motor directly to the propelling system. An internal combustion engine, on the other hand, will not run below a minimal revolutions per minute (RPM) and is also limited with respect to the maximal RPM it may attain. Therefore, the transmission used with such engines requires a clutching mechanism in order to allow the internal combustion engine to run while the vehicle is idle. Furthermore, the transmission must allow ratio changes between the engine output and the propelling system input, as high torque is typically required initially to propel the idle vehicle forward, to the detriment of the vehicle speed. Thereafter, lower torque is supplied for higher speed.
There are generally two main types of transmissions for internal combustion engine vehicles in the automotive industry: the discontinuous ratio transmission and the continuously variable transmission (CVT). The difference between the two types of transmission is comparable to the relation, in mathematics, between integers and real numbers. There are five integers comprised between 1 and 5 inclusively, whereas there is an infinity of real numbers between the same interval. The translation from an integer to the next integer implies a jump, a discontinuity. A discontinuous ratio transmission has such jumps. For instance, a five-speed vehicle has five different ratios, the ratio being the rotational speed at the inlet divided by the rotational speed at the outlet of the transmission. On the other hand, CVT's have an infinite ratio of speeds between inlet and outlet of the transmission, extending between a minimal ratio and a maximal ratio.
Discontinuous ratio transmissions are found on most cars, as they are highly efficient (in the vicinity of 95%) and highly reliable as there are no efficiency losses due to slip or overheating, and these transmissions are closed from water and dust damage. On the other hand, the discontinuity between the speed ratios and the necessity for clutching to switch speeds are major inconveniences. There is a loss in engine power, although small, when switching from one ratio to another. These transmissions also are more complex and require synchronization between the ratio changes. Furthermore, in difficult conditions, driver ability comes into account.
One type of CVT, the toric-drive transmission, involves a drive disk and a driven disk adjacent to one another, and shaped so as to form together a torus-shaped cavity. Rollers are positioned in the torus-shaped cavity so as to transmit motion from the drive disk to the driven disk. The input-to-output ratio changes as a function of the orientation of the rollers with respect to the disks, but is continuous. With CVT's, the change of speed and ratios is effected without discontinuity. The CVT's are also very flexible in allowing to optimize the use of the engine to which they are connected. However, CVT's are typically less energy-efficient than discontinuous ratio transmissions. For instance, in some type of toric-drive transmissions, actuation is required to displace rollers between the drive disk and the driven disk to change orientation, and hence vary the input-to-output ratio. More specifically, a translation of the rollers is caused to initiate a change in orientation to change the input-to-output ratio, whereby a non-negligible amount of actuation is used to cause the translation.